Radiation curable compositions are known to be useful, for example, in inkjet ink applications. In general, ultraviolet light (UV) inkjet inks are acrylate-based formulations with high viscosity (e.g., greater than [>] 50 millipascal-second [mP-s] at 25° C.). These inkjet inks need special printheads with heating elements to heat the ink formulation and simultaneously lower the viscosity of the ink formulation during jetting. Epoxy-based UV inkjet inks are also known in the art. At high temperature (e.g., >40° C.), acrylate-based and epoxy-based inks can gel; and gelling can clog printhead channels and thus render the printheads inoperable. Therefore, there is a need in the ink industry for a heat stable ink. Heat stability (e.g., exhibiting little to no change in viscosity) of an ink is a requirement especially for an ink that exhibits a long shelf life (e.g., no change in properties for 6-12 months).
Curing of acrylate inks is known to be inhibited by the presence of oxygen in air. In order to overcome oxygen inhibition of the free radical reaction, a higher level (e.g., 5 weight percent [wt %] as opposed to 0.5 wt %) of photoinitiator in an ink formulation is required. Epoxy inks are not inhibited by the presence of oxygen in air, and shrinks less than acrylate inks that lead to improved adhesion. Epoxy inks also have the desirable property of “dark cure”, whereby the system continues to react even after the light has ceased. Therefore, an unmet need in the ink industry is a UV ink formulation that is not inhibited by oxygen during curing/polymerization.
In the ink industry, the development of higher resolution inkjet printheads requires inks having a low viscosity and high thermal stability. Solvents are not preferred in ink jet formulations since the solvents need to be evaporated and release volatile organic compounds into the atmosphere. At present, an ink having a low viscosity is obtained by increasing the temperature of the ink and the printhead. This temperature increase can adversely affect the stability of the ink and the operation of the printer. The use of inks having low viscosity at ambient temperature eliminates the need for the use of heated printheads, improves cost effectiveness, and the environmental sustainability of the printing operation. The prior art fails to describe using a divinylarene dioxide such as divinylbenzene dioxide (DVBDO) in an inkjet ink curable composition having a high heat stability of, for example, above about 40° C. Therefore, an additional need in the ink industry is for a heat stable pigment UV inkjet ink having an initial viscosity of less than (<) 50 mPa-s. Preferably the inks comprise pigments such as cyan or magenta.